Computer memory arrays on chips involve a very large number of individual cells. For dynamic random access memories, the number of cells is very large. As a result, even small defect rates arising out of the manufacturing process result in an unacceptably low yield. Test procedures are applied to DRAM chips, usually on wafer-by-wafer basis. Every chip on each wafer is tested on specialized equipment, which identifies the locations of defective cells. Location information is then supplied to a controller for a laser repair device, which achieves a hardware fix. The repaired wafer is then tested again.
Such test and repair procedures are expensive because of the need to employ specialized test and repair equipment.
In SRAM chips, and other chips with embedded logic, repairs are not ordinarily carried out. The size of arrays in SRAM chips and other such chips has been small enough that, even without repairs, acceptable yield is obtained. Also, because SRAM chips are generally more specialized and manufactured in smaller quantities, the cost of configuring laser repair machines must be averaged over a relatively small number of wafers, when compared to DRAM chips.
In chips with embedded memories, it has become possible to have test procedures carried out by logic on the chip, known as built-in self-test units. The built-in self-test units for SRAM chips carry out a verification process resulting in a simple indication of whether there is a defect in the memory array. As defective chips are simply discarded, no additional information is required.
However, array size in SRAM chips is steadily increasing. Accuracy in manufacturing techniques is not increasing sufficiently rapidly to maintain yields. Furthermore, additional components, which were formerly in separate devices, are also being added to SRAM chips. The added components increase functionality of the chips, and are sometimes referred to as a system on a chip. These devices mean that individual chips are much more expensive, making discarding faulty chips undesirable.